In 3GPP, which is an organization that sets standards for third generation mobile communications systems, a study collectively termed as LTE (Long Term Evolution) has been conducted for achieving a drastic improvement of transmission speed and reduction of transmission delay in a radio access network (RAN); and formulation of standard specifications of the elemental technologies related to the study has been in progress.
As shown in FIG. 5, the radio access network (E-UTRAN: Evolved Universal Terrestrial RAN) in an LTE mobile communication system is configured of a mobile station UE (User Equipment) and a radio base station eNB (E-UTRAN Node B). This system is configured in such a way that the mobile station UE and the radio base station eNB communicate with each other via a radio link (RL).
Moreover, each of the mobile station UE and the radio base station eNB is configured to terminate an RLC (Radio Link Control) sublayer, a MAC (Medium Access Control) sublayer and a physical (PHY: Physical) layer.
In addition, a transmitting-side apparatus (mobile station UE or radio base station eNB) is configured to perform RLC processing, MAC processing and PHY processing sequentially for data to be transmitted and then to transmit the data as radio signals from a radio unit.
Meanwhile, a receiving-side apparatus (mobile station UE or radio base station eNB) is configured to extract the transmitted data by performing PHY processing, MAC processing and RLC processing sequentially for the radio signals received by a radio unit. Here, the data to be transmitted includes user data (U-plane data) generated by an application or the like used by the user, and control data (C-plane data) used in controlling the mobile communication system, such as RRC (Radio Resource Control) signaling and NAS (Non Access Stratum) signaling.
Moreover, this system is configured in such a way that RLC retransmission control processing is performed between the RLC sublayer of the transmitting-side apparatus and the RLC sublayer of the receiving-side apparatus, and that HARQ (Hybrid Automatic Repeat Request) retransmission control processing (MAC retransmission control processing) is performed between the MAC sublayer of the transmitting-side apparatus and the MAC sublayer of the receiving-side apparatus.
Hereinafter, with reference to FIG. 1, a description will be given of window control and retransmission control processing in the RLC sublayer of the transmitting-side apparatus on the basis of acknowledgement information from the RLC sublayer of the receiving-side apparatus.
As shown in FIG. 1, when a transmission trigger #1 of a STATUS-PDU (acknowledgement information) occurs, firstly, the RLC sublayer of the receiving-side apparatus transmits a STATUS-PDU (ACK=10) to the MAC sublayer of the receiving-side apparatus in accordance with a current RLC-data-PDU reception state, the STATUS-PDU being for notifying an RLC-data-PDU expected to be received next.
Here, the RLC sublayer of the receiving-side apparatus is configured to transmit the STATUS-PDU (ACK=10) in a state where an RLC-data-PDU with a sequence number #10 is received without detection of a loss.
Secondly, the MAC sublayer of the receiving-side apparatus transmits the STATUS-PDU (ACK=10) to the MAC sublayer of the transmitting-side apparatus by using HARQ retransmission control processing.
Thirdly, the RLC sublayer of the transmitting-side apparatus updates the lower limit value and the upper limit value of a transmitting-side window to “11” and “522”, respectively, in accordance with the STATUS-PDU (ACK=10) received from the MAC sublayer of the transmitting-side apparatus. Here, the window size of the transmitting-side window is assumed to be “512”.
Moreover, since the lower limit value of an acknowledgement information reception window managed by the RLC sublayer of the transmitting-side apparatus is equal to the lower limit value of the transmitting-side window, the RLC sublayer of the transmitting-side apparatus updates the lower limit value of the acknowledgement information reception window to “11” as well in accordance with the STATUS-PDU (ACK=10) received from the MAC sublayer of the transmitting-side apparatus.
On the other hand, the upper limit value of the acknowledgement information reception window managed by the RLC sublayer of the transmitting-side apparatus is set to a value of the latest sequence number of a packet transmitted to the MAC sublayer from the RLC sublayer in the transmitting-side apparatus. Thus, the RLC sublayer of the transmitting-side apparatus does not update the upper limit of the acknowledgement information reception window even when receiving the STATUS-PDU (ACK=10).
Next, when a transmission trigger #2 of a STATUS-PDU (ACK=100) occurs, the aforementioned mobile communication system operates in the same manner, and the lower limit value and the upper limit value of the transmitting-side widow are updated to “101” and “612”, respectively. Furthermore, the lower limit value of the acknowledgement information reception widow managed by the RLC sublayer of the transmitting-side apparatus is updated to “101”.
Here, in an IMT-2000 mobile communication system, as shown in FIG. 2, the transmitting-side apparatus is configured in such a manner that the MAC sublayer performs reordering processing for received RLC-PDUs (actually, MAC-PDUs to which the RLC-PDUs are mapped) including STATUS-PDUs (acknowledgement information), and then transmits the STATUS-PDUs (acknowledgement information) in the order of the sequence numbers to the RLC sublayer.
Accordingly, when the communication state is normal, the sequence number of a packet included in the STATUS-PDU (acknowledgement information) received by the RLC sublayer of the transmitting-side apparatus is always assured to be a value within the range of the acknowledgement information reception window managed by the RLC sublayer of the transmitting-side apparatus.
Then, if the sequence number of a packet included in a STATUS-PDU (acknowledgement information) received by the RLC sublayer of the transmitting-side apparatus takes a value outside the range of the acknowledgement information reception window managed by the RLC sublayer of the transmitting-side apparatus, the RLC sublayer of the transmitting-side apparatus determines that the communication state is abnormal and thus initiates RLC reset processing.
Here, since the RLC sublayer of the transmitting-side apparatus does not transmit an RLC-PDU (data packet) until the RLC reset processing is completed, it is to be noted that initiation of the RLC reset processing notably degrades the communication quality.    Non-Patent Document 1: RLC Specification of IMT-2000: 3GPP TS 25.322 V6.9.0 (2006-09)    Non-Patent Document 2: LTE Stage 2 Specification: 3GPP TS 36.300 V8.0.0 (2007-03)